Natural polymeric materials are polymeric materials from biological systems or derived from biological systems. Examples of natural polymeric materials include; polysaccharides, such as cellulosic materials and starch based materials; protein based materials; polymers derived from monomers that occur in biological systems but are prepared using synthetic methods; and polymers produced by micro-organisms.
Polysaccharides constitute the major proportion of plant structural material and include cellulose, and its derivatives, starches, pectins and hemicelluloses. Cellulose is the most abundant polysaccharide and constitutes one half of the weight of perennial plants. Cellulosic materials such as plant material, wood, wood and wood-based products, paper and other substances containing natural cellulose-based fibres are one of the most important material resources and are used in a wide range of objects including buildings and their components such as cladding/sidings, window frames, doors and door frames, decking and others, furniture, clothing and paper products. Wood is not only used in its raw form but also in the form of the fibres, strands, or chipped wood are used for making pulp paper, fibreboard, plywood, oriented strand boards, laminated board, pellets, composite materials with either natural or synthetic polymeric matrix or inorganic matrix and/or binders and other products known to those skilled in the art.
The efficient and durable bonding or contact of other types of organic and/or inorganic materials such as paints, adhesives, synthetic resins, metallic coatings, electroconductive or charge transfer materials, UV-, IR-, or MV absorbing materials, inks, preservatives and composite components to natural polymeric materials is critical to the performance and longevity of these products in a number of important industrial applications. In these applications they may be applied to the surface or to the bulk of these products.
Vegetable products based on natural polymers such as cellulosic materials are often difficult to wet and bond. There use in many other specific functions is also problematic because of low surface energy, incompatibility, chemical inertness, or the presence of contaminants and weak boundary layers. The lack of adequate adhesion at the substrate/adherent and/or reinforcement/matrix interfaces often results in poor material performance and limits the possible applications of the products made with these materials. Effective surface treatments are frequently required to overcome one or more of the above mentioned difficulties in order to achieve controlled or maximized the product or composite performance and controlled level of adhesion with paints, adhesives, functional coatings, bio-active materials, or other materials.
An example of a specific application is the electrostatic painting process on cellulosic substrates which may involve organic solvent- or water-based paints or those suitable for powder-coating. The electrostatic painting process has advantages over conventional painting process as up to 80% less paint is used and the VOC can be greatly reduced when less paints are used. To satisfy the electrostatic painting requirements, the surface/interface layer of polymer based materials must possess electrical conductivity and good adhesion to both substrate and paints.
The properties of wood have a significant effect on its ability to bond with paint and other materials. The dimensional changes at the late-wood-earlywood interface can cause cracks in film-forming finishes at this zone. Paint failure on latewood often begins with these cracks. If the bands of latewood are narrow enough, as in slow growth trees, the stresses are decreased and there is less tendency for paint to crack or peel than on the wide latewood bands. Wide latewood bands are normally absent from edge-grained cedar and redwood improving the paintability of these species. It is well established that wide latewood bands on softwoods give a surface that is difficult to coat or paint or to provide other type of finishing.
Water also causes peeling of paint. Even if other factors are involved, water accelerates paint adhesion degradation. If the moisture content of the wood exceeds 20% when the wood is painted, the risk of blistering and peeling is increased.
Although the erosion of a wood surface through weathering is a slow process, the chemical changes that occur within a few weeks of machined wood storage or outdoor exposure can drastically decrease the adhesion of adhesives or paints subsequently applied to the stored or weathered surface. Wood stored for excessively long times or badly weathered, cannot hold adhesive paint very well. However, even over a period of only two to three weeks the wood may appear sound and much the same as unexposed wood but when smooth-planed boards that have been preweathered for 1, 2, 4, 8 or 16 weeks then adhesively bonded or painted, the adhesive or paint drastically losses adhesive strength after four weeks of preweathering. For panels preweathered for only one week, the paint may start to peel more quickly than unweathered wood.
Paint applications are especially susceptible to performance failures when surface checking of the wood substrate occurs. These checks initiate cracking and peeling of the coating. Kiln drying dramatically decreases this condition but is not always desirable or convenient.
It is desirable to modify the wettability of natural polymeric surfaces in many practical applications. The surfaces of articles of natural polymeric materials and their composites may also be required to exhibit a specific level or gradient of wettability by organic and/or inorganic liquids or vapours of these liquids. Depending on specific end-applications, the liquid phase or condensate may be required to form a uniform film (requiring a hydrophilic film for aqeous compositions) or alternatively, it may be required to bead-up on an unwettable liquid-repellent surface (a hydrophobic surface for aqeous compositions). It is also possible that in some instances, an intermediate level of wettability is desirable. The surface/interface with a specified or well defined wettability must overcome the adverse effects of polymer surface restructure and continuous washing cycles to remain effective.
Cellulosic materials are also used in the manufacture of composites in the form of sheets, particles or fibres, strands, woven fabrics with a synthetic resin or natural polymer-based resin or an inorganic material as a matrix or binder and optionally other filler materials. Such products have a tendency to breakdown particularly in the presence of moisture and fluctuations in temperature.
The durability of adhesion to a solid material or composite-based product or assembly subjected to high humidity, fluctuation of temperature and UV irradiation are very critical when the products are for out door application, such as unpainted or painted external components used in the building or automobile industries. The hydrothermal stability of the interface/interphase often determines the success of the surface modification process and the ultimate product performance.
Natural or synthetic polymer based materials are often required to provide surface properties such as good adhesion or chemical linkage to another material and at the same time provide a diverse range of physio-chemical properties such as strength, flexibility or elasticity, inertness or reactivity, electrical or heat conductivity, UV- or IR energy absorbance, moisture or vapour, barrier properties, biocide or fungicide functions, or wettability for various applications.
The performance and adhesion of materials such as organic, inorganic and metallic coatings, adhesives, preservatives or reinforcing resins based on natural and/or synthetic polymers and their inorganic counterparts to natural polymeric materials, particularly cellulosic materials has therefore been the subject of considerable research and development.
We have now found that the bonding of materials to natural polymeric material such as cellulosic materials can be substantially improved by modifying the natural polymer based material using certain chemicals.